I.— PHYSIOLOGY 179 



degree of shock or permanent increased capacity of the circulation from 

 the absorption of toxic products. As a result, the increased cardiac 

 activity more than balances any increased flow in the veins, and the venous 

 pressure may actually fall. Commonly it remains unchanged in such 

 experiments. However, if the animal is not subjected to any severe 

 operative procedure, a small rise of venous pressure is the rule. Perhaps 

 I should say that several workers using the Fick method have shown an 

 enormously increased output of the heart when the impulses from the 

 carotid sinus are cut off. 



In doing such experiments we must attempt to imitate physiological 

 possibilities. If, for example, we cut off all the depressor reflexes com- 

 pletely and suddenly, there is such an enormous rise of venous pressure and 

 arterial pressure that the heart may fail and the cardiac output be reduced. 



What we can imagine happens in exercise or emotion is, then, that just 

 as the vagus restraint of the heart becomes reduced, so also the depressor 

 restraint of the vessels becomes reduced, more blood is thrown into the 

 circulation and is dealt with by the heart, which at the same time 

 increases both its rate and its output per beat. It is to be anticipated 

 that we shall eventually get evidence that the extent of the activity of the 

 vasodilator reflexes varies in different animals just as the activity of the 

 vagus varies. 



The sympathetic and adrenaline. — All the mechanisms which I have 

 described are probably still further enhanced by the vasoconstrictor 

 action of the sympathetic and the action of adrenaline, which is apparently 

 secreted whenever the emotional stress of the occasion is sufficient. 

 Adrenaline in physiological amounts constricts the vessels of the skin 

 and splanchnic region and dilates the vessels of the muscles. Here I 

 should like to emphasise that probably the physiological dose of adrenal- 

 ine is minute, and may even be insufficient to raise the blood pressure. 

 Certainly the dilatation of muscle vessels is not a result of the rise of blood 

 pressure which may occur, for it can be shown that the dilatation occurs 

 with doses which do not raise the arterial blood pressure. An increased 

 blood flow through the limbs can also be shown to be brought about by 

 doses which do not raise the blood pressure. In such circumstances the 

 constriction just counterbalances the dilatation. Why adrenaline should 

 constrict some blood vessels and dilate others is a major problem in the 

 study of the circulation. Since so far as we know the vessels themselves 

 have the same structure in different parts of the body, we must assume 

 that the difference is due to the different environment. I had hoped by 

 this time to have obtained some definite evidence on this point, but so far 

 the experiments have not been completely successful. 



It is interesting to observe the effect of adrenaline on the depressor 

 reflexes. If the hormone is injected it is found that some minutes after- 

 wards, even after the usual rise of blood pressure has passed off, it is not 

 possible to affect the heart by a degree of stimulation of the vagus which 

 was previously effective, and at the same time the effects of cutting off the 

 impulses from the carotid sinus are markedly reduced or completely 

 abolished. This, of course, is exactly what would be expected if ad- 

 renaline were secreted in the same circumstances in which the action of 

 the depressor reflexes and the vagus are reduced, as in exercise. 



